Printed Electronics (PE) are highly desirable because they can be printed on-demand, anywhere and anytime; are scalable (e.g. into large formats like posters or wallpaper); and are inexpensive such that they can be used everywhere print media is used (i.e. with a unit cost of the order of cents such that they are considered to be disposable). Furthermore, one of the key technological advantages of Printed Electronics (PE) is its realization on flexible substrates (e.g. plastic) that can be moulded or bent in use or to be fitted into awkward or uneven spaces (see references 1-5 at the end of the specification).
Printed electronic printing technologies can, in general, be classified as either ‘Subtractive’ or ‘Additive’ processes. A Subtractive process involves a series of additive (deposition) steps followed by subtractive (etching, lift-off, etc.) steps (see references 6-7 at the end of the specification). In contrast, an Additive process involves deposition-only steps and it is therefore also known as a ‘Fully-Additive’ printing process.
Subtractive processes are dominant due to superior performance of the resulting devices when compared to those made using the Fully-Additive process. Accordingly, reported PE circuits and systems are generally realized using Subtractive processes (see references 1-4 at the end of the specification). However, disadvantages of the Subtractive process include the fact that it is not environmentally friendly (due to the use of corrosive chemicals); it is not on-demand (with attendant slow throughput and long processing times); and it is relatively expensive (requiring complex equipment and infrastructure and high wastage of chemicals, e.g. due to etching/lift-off procedures). These disadvantages contravene the desirable aims associated with printed electronics, as outlined above.
On the contrary, a Fully-Additive printing process is low cost, includes simple processing steps (i.e. no subtractive steps), can be performed on-demand (printing anywhere, anytime), with high throughput (quick printing) and scalability (large format printing).
However, irrespective of the printing process employed, one critical drawback with printed electronics is that the characteristics of printed electronic devices and sensors (including transistors, capacitors, resistors, etc.) vary significantly when the substrate is bent, resulting in intractable variations in the device performance and, in many cases, it is not possible to distinguish if the change is due to a sensed parameter or due to bending. In some cases, the bending (which may be convexly/outwardly or concavely/inwardly) may result in device, sensor, circuit or system failure (see references 8-9 at the end of the specification). Furthermore, in the context of synchronous-logic circuits, bending may lead to increased delays that may exceed a time constraint; leading to the need for asynchronous-logic circuits.
There are two reported methods (see references 13-14 at the end of the specification) to accommodate the change of the characteristics of electronic devices due to bending, and both methods have obvious drawbacks. In particular, in reference 13, the devices need to be located in an area where the bending is minimal. The major drawback of this method is the requirement of a priori information about the nature of the bending before the printed electronic devices are made and this information is generally unknown in advance of the manufacture and use of the device. In reference 14, another substrate layer is placed on top of the printed electronics such that the physical deformation of the printed electronics is reduced. However, this method undesirably doubles the overall thickness of the device and, in some sense, defeats the flexibility advantage of flexible substrates. In summary, these methods are mechanical solutions that may, at best, reduce the variation in the PE characteristics due to bending but will not eliminate it to a first order and therefore some ambiguity will remain. Accordingly, the above methods are largely insufficient and ineffective at addressing the problem associated with bending of printed electronic devices.
U.S. Pat. No. 3,723,635 discloses a method for forming double-sided flexible printed circuits wherein two sets of terminals are initially formed on the same side of a substrate, with each set being separated by a distance sufficient to allow the resulting substrate region there-between to be bent around and secured to a rigid support member. The two sets of terminals are therefore ultimately positioned on opposite sides of the support member. However, the presence of the rigid support member means that the resulting product is not flexible, at least in the region of the support member, even though the initial substrate is flexible.
U.S. Pat. No. 9,076,822 discloses a generally rigid carrier substrate—on a first side of which is mounted a first flexible substrate and on a second side of which is mounted a second flexible substrate. A first electronic device is deposited on the first flexible substrate and a second electronic device is deposited on the second flexible substrate. However, as above, the presence of the rigid carrier substrate means that the resulting product is not flexible, at least in the region of the carrier substrate, even though the first and second substrates are flexible.
It is therefore an aim of the present invention to provide an improved method of fabricating an electrical circuit assembly on a flexible substrate that helps to ameliorate one or more of the above problems.